This invention relates to the production of multiple component composite structures by spray depositing mixtures of a resin and catalyst along with a lightweight, low cost, dry particulate filler material such as hollow glass microspheres and other hollow bodies having a specific gravity of less than about 1.00, and optionally a fibrous reinforcing material such as glass fibers or carbon fibers, for example.
It is known in the prior art that composite structures made by spray depositing mixtures of a resin and catalyst along with a reinforcing material such as fiberglass are strong but not necessarily very rigid. Stiffness is a function of thickness and in order to attain a required degree of rigidity, it has become the practice to make the composites thicker by adding extra resin and reinforcing material. Unfortunately, this greatly increases the cost and weight of the composite. Since rigidity or stiffness is related to the composite thickness, what is clearly needed is a matrix extender system which will increase the composite thickness but which at the same time is lightweight and inexpensive.
Microspheres, in the form of lightweight hollow glass or silicate spheres, are ideal materials to achieve extension in many reinforced composite structures. Microspheres are very lightweight generally having a specific gravity of less than about 1.00, they are inexpensive and significantly reduce exotherm shrinkage and cracking when used as a filler or extender in a composite structure.
Heretofore, attempts to employ hollow microspheres in the production of reinforced composite structures have not proven to be successful. Until now, the only way of placing the hollow spheres into the composites has been by premixing the spheres into the resin. This has essentially two processing problems, first, since the spheres are considerably lighter than the resin, they tend to rise to the surface of the resin and will not remain in suspension. Hence, it is very difficult to maintain an even distribution of the spheres within the resin. The second problem is that when the hollow spheres are finally passed through the resin pump, high pressures are attained which tend to collapse the spheres and render them ineffective. This collapsing of the spheres can be somewhat eliminated by increasing the wall thickness of the spheres but this unfortunately diminishes the cost effectiveness of the spheres to the point where it is more practical to employ the heavier solid fillers, such as calcium carbonate, for example.
It has also been proposed in the prior art to inject a dry particulate filler material into a resin- and catalyst-containing stream such as that produced by a conventional spray gun and then depositing the resulting mixture onto a mold or substrate for forming the composite structure. Such a system is disclosed, for example, in U.S. Pat. No. 3,927,833 issued to Harrison, et al on Dec. 23, 1975. This device employs a spray gun and a pair of delivery tubes disposed adjacent to the spray nozzle for injecting the dry particulate filler material directly into the resin- and catalyst-containing stream at a fixed angle of about 90 degrees with respect to the longitudinal axis of the spray pattern. The particles of filler material are stored in a container or tank equipped with a venturi pump and are carried to the spray gun in a flowing stream of compressed air. Although such a system may be suitable for injecting relatively heavy granules or particles of filler material such as clay, sand, talc, mica and the like, into the resin- and catalyst-containing stream, it is totally unsuitable for injecting relatively lightweight, hollow microspheres and other hollow bodies having a specific gravity of less than about 1.00. These lightweight filler materials tend to be cast outwardly away from the high velocity stream rather than intermixing therewith when injected at an angle generally perpendicular to the longitudinal axis of the spray pattern. Unfortunately, the atmosphere surrounding the spray gun fills with a cloud of hollow spheres, wasting the spheres and creating an unsafe, contaminated environment. Moreover, there is no provision in this system for the operator to control the concentration of the dry particulate filler material fed to the spray gun and eventually deposited in the composite structure.